Tau is a microtubule associated protein found primarily in neuronal axons. Physiological phosphorylation of tau regulates the dynamics of the association of tau with tubulin, and thereby microtubule stability (Mazanetz. M. P. and Fischer, P. M. 2007. Nature Reviews 6:464-479). The stabilization of the microtubules in axons ensures that maintain their function for axonal transport, growth and branching (Bulic, B et al., 2009 Angew. Chem. Int. Ed. 48:2-15). Hyperphosphorylation and misfolding of the tau protein is thought to be the causative factor in abnormal intracellular aggregation leading ultimately to neuronal dysfunction. Protein aggregates have been found to be toxic to neurons.
Abnormal intraneuronal tau aggregation has three basic pathological manifestations; neurofibrillary tangles (NFT's), neuropil threads (NT's) and the argyrophilic dystrophic neurite plaques (Braak, H and Braak, E, Neurobio. of Aging. 1997 18(4):351-357). Structurally, the NFT's are principally comprised of paired helical filaments (PHF) comprised of two filamentous tau proteins twisted around one another with a crossover repeat of 80 nm and a width of 8-20 nm (Li, D., et al., 2008. Computational Biology 4(12) and Kidd, M 1963 Nature, 197:192). There are six stages (Braak stages I-VI) of tau deposition in the brain, which progress temporally at defined anatomical locations with the initial stages characterized primarily by the deposition of NFT's and NT's and the secondary stages further accompanied by NP (Braak, 1997). In AD and other neuropathies, Braak's stages correlate well with clinical disease progression as demonstrated by increasing cognitive dysfunction. Severe cortical destruction which occurs around stages III-IV coincides with the first manifestations of the clinical onset of AD. Although no tau mutations have been identified in AD there is a strong correlation between NFT density and cognitive decline in AD (Brunden, K. R., Trojanowski, J. Q., and Lee, V. M. 2009 Nature Reviews 8:783-93).
New biomarkers and models of their temporal characteristics are becoming even more useful for the diagnosis and characterization of AD (Jack et al., 2010. Lancet 9:119-28). Specifically, tau deposition is associated with neurodegeneration in AD and an increase in CSF tau is an important indicator of tau pathologic changes and correlates well with clinical disease severity. A decrease in FDG-PET correlates well with increased CSF tau and both are valid indicators of synaptic dysfunction (Jack et al, ibid). This model of biomarker ordering, especially in mildly cognitive impaired individuals, has important implications for clinical trials. Potential therapeutics could be more accurately assessed for efficacy is they are able to change the trajectory of cognitive deterioration and individuals might be more selectively chosen for trials (Jack et al, ibid).
Tau hyperphosphorylation is a common characteristic of a number of dementing disorders collectively known as tauopathies, some of which have disctinct tau pathology combined with other brain pathologies. Tauopathies include Alzheimer's disease (AD), Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and familial frontotemporal dementia/Parkinsonism linked to chromosome 17 (FTDP-17), amyotrophic lateral sclerosis/Parkinsonism-dementia complex, argyrophilic grain dementia, dementia pugilistic, diffuse neurofibrillary tangles with calcification, progressive subcortical gliosis and tangle only dementia. (Spillantini, M G and Goedert M, 1998 Trends Neurosci. October 21(10):428-33). In AD, tau pathology is typically limited to the neurons while other tauopathies can pathologically exhibit both neuronal and glial tau deposition (Higuchi, M, et al., 2002. Neuropsychopharmacology: The Fifth Generation of Progress, Chapter 94: Tau protein and tauopathy).
It has recently been postulated that tau protein may link Parkinson's and Alzheimer's disease (Shulman, J. M. and DeJager, P. L. 2009 Nature Genetics 41(12): 1261-1262). This study examined whether any genome wide association occurs between the two diseases and found that three genes and two new loci were linked to increased susceptibility.
It is presently not known if tau is a causative factor in disease but it is likely that either a loss or gain for function results in pathology. In FTLD17, a missense mutation affects the alternative splicing of tau resulting in the disruption of the ratio of the 4R to 3R tau isoform. More of the 4R isoform with an extra repeat of the microtubule binding region may lead to overstabilization of the microtubules resulting in disease. Other post-translational events such as alterations in kinase activity and glycosylation could also cause hyperphosphorylation and result in disease or alternatively proteolytic cleavage could produce truncated tau products more inclined to aggregate (Brunden, ibid).
Recently tau toxicity has been re-emphasized as an important therapeutic target in neurodegerative tauopathies (Keystone Symposium, March 2009). Routes for developing therapeutics are either directed to inhibiting tau-phosphorylation kinases or seeking compounds effective in the modulation of tau aggregation and/or the dissolution or disruption of tau aggregates which may prove equally useful or more specific for the alleviation of tauopathies (Rafii, M. and Aisen, P. 2009 BMC Medicine 7:7). A recent paper surveyed the efficacy of several classes of compounds for their ability to prevent tau aggregation and disaggregate pre-formed tau fibrils (Bulic et al.). Although there are general concerns regarding the toxicity of disassembled fibrils, Bulic et al., were able to show that reversing tau aggregation resulted in increased cell viability.